A method of the type to which the present invention is directed is disclosed in European Patent Disclosure EP 0 112 645 B1. The device disclosed therein for performing a method of the type here under consideration includes an electronic evaluation unit with a memory member and a display member for the measured intensity distribution Z of the radiation, where a screen plate with a plurality of evenly spaced screen openings is provided between the detector and the sample.
The detector provided therein is described as consisting of an ionization chamber. A decoding circuit is indicated to be preferably provided for evaluation, which permits the identification of that intersection point or assigned screen opening beneath which radiation was detected.
However, ionization chambers are not usable for detecting weak radioactive radiation (T. R. Roberts, "Radiochromatography" in Journal of Chromatography Library, Vol. 14, Elsevier Scientific Publishing Co., 1978), so that the practical application of such a location-sensitive operating measuring system is limited.
In a device of the type here under consideration, an intersection point between counting wires which extend in x- and y-directions is associated with each screen opening in wire levels disposed on top of each other in the ionization chamber where, by means of the above mentioned decoding of tile information received, the counting rate of the partial area of the sample associated with this screen opening can be detected or measured. Thus, the known device operates as an m.multidot.n-multi-counter (corresponding to m.multidot.n-counting wire intersection points) which, as a function of the shape of the screen opening, scans in a meandering fashion over the sample surface to be measured and in the course of which detects m.multidot.n image points during each partial measurement (FIG. 1b of EP 0 112 645).
The local resolution in the x-plane as well as in the y-plane which can be obtained with this operating principle (matrix-like disposition of screen openings and meander-like scanning movements) is achieved here by the size of the screen openings in the coordinate direction. If a very high local resolution is required, such as with 32-P-marked DNS sequence gels and blots, where resolutions on the order of 0.3 mm are desirable as typical values, a very large number of screen openings is needed, with correspondingly expensive decoding circuitry and a plurality of evaluation wires, or a correspondingly higher number of partial measurements with a very slow and structurally expensive scanning movement and a very long measuring time resulting therefrom.
With special applications, such as the already mentioned measurement of DNS sequence gels or blots, measurements which are only location-sensitive in one direction (one-dimensional) are sufficient.
For this special case, a device of the type under consideration can be provided with slit-shaped screen openings, the width (x-direction) of which corresponds approximately to the width of the bands of a DNS sequence gel to be measured. By means of the still required one-dimensional scanning movement it is then possible, for example, to detect samples in the form of adjoining bands simultaneously through a scanning movement in the direction of the bands (column 12, line 64, to column 13, line 5, of EP 0 122 645 and FIG. 1 thereof).
The simultaneous detection of bands which adjoin perpendicularly to the scanning direction, however, can no longer be considered to represent a location-sensitive measurement in that direction. A "location resolution", which in this case is at best fictional, of such a measurement has a value which is inferior to the length of the slits in the x-direction, which results in coarse averaging of the radioactivity distribution above the slit.
The performance price which has to be paid by such a device for simplified linear scanning, therefore, is necessarily the loss of location sensitivity in the direction perpendicularly to the scanning direction, so that with this kind of operation such device can no longer be used for two-dimensionally sensitive measurement. Because of this, detailed structures, such as arc-shaped radioactivity patterns which for example occur in DNS sequence gels, are necessarily lost.
Furthermore, such limitation to a one-dimensional scanning movement is only justifiable if the focus of the bands to be measured does not vary in the x-direction. In actuality, the two effects mentioned (arc-shaped radioactivity pattern with temporal offset) occur quite often together. In such cases it would not even be possible to perform a meaningful association of the measured mean values to the respective bands, and the measurement would be unusable. Finally the device, reduced by using the slit screen with a one-dimensionally location-sensitive operating device (and therefore no longer in accordance with the type of device under consideration), is therefore only usable in special exceptional cases (ideal extent of the bands).
Another known type of device for the two-dimensional measurement of radioactive radiation includes multi-wire proportional counters, such as described in German Published, Non-Examined Patent Application DE-OS 37 35 296 and counterpart U.S. Pat. No. 4,965,861. A proportional counting tube of this type which operates in a location-sensitive manner has three parallel wire levels, namely an anode wire level and two readout levels (the latter designated D.sub.x and D.sub.y). As a rule, one readout level is located above the anode level, the other below it.
The wires of the readout levels can supply pulses induced therein to delay lines, namely one for the x-plane and one for the y-plane. Each supplied pulse moves from the place of introduction towards both ends of the delay line. Rapid response amplifiers are located at the ends of the delay line and the time difference between the arrival of each of the pulses at the two ends is measured. By the provision of an additional constant delay at one side of the delay line, the time period to be analyzed is made to lie between zero and twice the total delay of the delay line. This time period can be directly digitized by a time digital converter (TDC), or it is first changed into an analog value and then digitized by means of a pulse analog digital converter (ADC).
It is possible to realize a local resolution of approximately 0.5 mm (with a perpendicular light incidence as the ideal state) with such a two-dimensional proportional counter. However, this can be limited because of various nonlinearities and distortions in the electronic components. This hardware-created lower resolution limit can also be called the "inner resolution". In actuality, values for the local resolution of approximately 1 to 2 mm are the result of the mentioned non-linearities and the, as a rule isotropic, emission of the radiation from the sample. Collimators of different types are known for increasing the local sensitivity. For example, German Letters Patent No. 39 15 613 uses a screen with a plurality of screen bores, with the aid of which it is possible to achieve a local resolution in the range of approximately 1 mm with isotropic radiation incidence. To go below this value is essentially a matter of the technological limits in controlling the screen bore diameters. With a bore diameter of 0.5 mm in accordance with the above mentioned patent, in case of a specimen plate of 20.times.20 cm the result is already 80,000 bores in a collimator plate of this type.
Thus, the situation is that the two-dimensional, location sensitive measurement of flat samples by means of the known, two- dimensionally operating proportional counting tubes (even with the aid of collimators) is possible only with limited local resolution; and with the device under consideration only with an extreme effort in respect to apparatus and/or time.